Vehicles today are increasingly equipped with electric drive motors, which, in addition to propelling the vehicle, can capture braking energy to charge a battery. Depending on how the vehicle powertrain is configured, this process, known as “regenerative braking”, can occur at the front axle, the rear axle, or both. There are other kinds of non-friction braking, for example, engine braking, which occurs when the compression of the engine provides a negative torque to the vehicle drive axles. Where the engine is only connected to one axle, as in a two-wheel-drive vehicle, or where the regenerative braking is only available at one axle, there may be competing interests between trying to brake in such a way as to maximize non-friction braking, for example, to maximize energy capture in a regenerative brake system, and more evenly distributing braking torque between the front and rear axles to provide better vehicle handling.
Adding complexity to the braking control system is consideration of the vehicle load. This may be of particular concern with commercial vehicles where the difference between the loaded weight and unloaded weight is significant. If, for example, a brake system is configured to maximize non-friction braking at the rear axle for the fully loaded vehicle, the brake system may over brake at the rear axle when the vehicle is unloaded. In addition, if the brake pedal travel is mapped the same for the loaded and unloaded conditions, the brake pedal may be “too sensitive” when the vehicle is in the unloaded condition—i.e., a very hard braking may occur for a very small amount of pedal travel. Conversely, if the brake system is configured to maximize non-friction braking at the rear axle for the unloaded vehicle, the brake system may not utilize all of the available non-friction braking—e.g., it may not capture all of the possible regenerative braking—when the vehicle is loaded. This may be due, in part, to the lack of sensitivity of the brake pedal, which now may need to be depressed so far as to engage the vehicle's friction brakes before all of the available non-friction braking energy is utilized.